The high cost of windows for optical guidance systems of missiles has long been a concern. This has been made more critical by current needs to obtain high transmission over more than one region of wavelength, severely limiting the choice of materials which can be used. For example, the Stinger POST and Stinger RMP systems both require windows which transmit not only in the infrared (IR) but also in the ultraviolet (UV) spectral range. This dual transmission need, along with other requirements, have resulted in the specification of a single material, calcium aluminate glass as the standard window. As the program matured, expected cost reductions were not achieved and a research and development program was initiated to examine the causes.
Not only has the specified calcium aluminate glass increased in cost, but the quality of that material continues to vary over time, resulting in problems which continue to effect yields. The fundamental purpose of the window is to provide a means for the sealed optical guidance system of the missile to "see" the target. For that reason the transmission of the window in the ultraviolet and the infrared spectral ranges is an overriding and crucial requirement. Calcium aluminate glass has marginal transmission at the extreme of both the ultraviolet and infrared spectral ranges. This is exacerbated by variations and problems associated with the calcium aluminate which material includes: failure of the window to meet transmission requirements for the ultraviolet and infrared wavelength regions, internal inclusions, bubbles and devitrification which result in scatter of wavelengths being received, internal striae (index variations in the glass) which result in distortion of the optical path, high refractive index of the glass material, requiring an antireflection (AR) coating for two widely separated wavelength regions to meet typical transmission requirements, and water solubility of the material which necessitates the inclusion of a coating to protect the calcium aluminate window in use, but which makes the glass difficult to fabricate because maximum polishing rates are obtained with water based slurries of the selected obtained polishing compounds which, unless used with great care, etch and stain the polished surface. This severe surface staining and surface etching reduces transmission and results in scatter, with the effect of scatter being intensified by the required combination antireflection AR and protective coating.
Further, calcium aluminate glass leads to frequent failures in burst and proof tests due to the material's wide variations in rupture modulus as defined in ASTM C 158. This is inherent in the glass due to the lack of a well ordered structure. Proof and burst tests comprise sealing the window onto an airtight chamber and applying a first specified pressure, known as the proof pressure, for a specified time and noting that the window withstands that pressure for that time. This is followed by applying a second specified pressure, the burst pressure, and noting if the windows fracture or break at that pressure. Also coloration results when the calcium aluminate glass is exposed to sunlight or UV radiation and causes corresponding reduction in the UV and visible transmission of the glass.
The physical and chemical properties of the calcium aluminate glass which are the cause of these problems and the cause of resulting process complications are inherent in the material and the best solution to the problems noted above is to develop an alternative material without such undesirable characteristics.
A range of oxide, halide, and sulfide materials which transmitted in the desired wavelength regions were surveyed and a number chosen for more detailed consideration. These included glasses as well as polycrystalline and monocrystalline materials.
The glasses of sufficient transmission in the UV/visible (wavelengths of 300-500 nm) and IR (wavelengths of 0.8-5.mu.m), including germanate and various fluoride based glasses, all lacked sufficient strength, i.e. a modulus of rupture less than 12,800 psi, to meet specifications. In addition, the wide statistical distribution of strengths in such glasses makes it impracticable and improbable that a glass window can meet the narrow range of proof/burst pressures specified without incurring some unacceptable percentage of continuing rejects and thereby complicating the quality assurance procedure and increasing costs.
A number of polycrystalline and monocrystalline oxides have sufficient transmission and strength to meet Stinger POST and Stinger RMP specifications. These include sapphire, spinel, yttria and ALON# material. These materials are available as follows: sapphire from Crystal Systems Inc. of Salem, MA; Spinel from Alpha Optical Systems, Inc. of Ocean Springs, MS; yttria from GTE Laboratories Inc. of Walthan, MA; and ALON.TM. material from Raytheon Co. of Walthan, MA.
All of such materials have a sufficiently high refractive index, i.e. above 1.50, and therefore require an anti-reflection coating to eliminate reflection loss. In addition, all require burst risers to reduce the burst pressures to be within the specified values. A burst riser can be defined as a circular pattern ground into the center of one window surface in a manner to induce a controlled stress in the window. A burst riser might also include any structural or mechanical variation in the glass, or possibly an inclusion, such as a foreign element or a variation in the material or chemical composition. With the exception of the costly single crystal sapphire, all exhibit transmission scatter problems with the problem in ALON.TM. material being particularly severe due to numerous gaseous inclusions at the grain boundaries.